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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

22                                         CITRIC ACID

the ash weighed.   If this exceeds 0-5%, a greater quantity of the acid is
burned and the ash tested for lead, copper and iron.

6.  Lead.—Of particular importance is the determination of the lead,
the presence of which in small quantities is clue to the crystallisation vessel.
Warington's colorimetric method is used :

A standard lead solution is prepared by dissolving about 50 grams of
ammonium citrate (puriss.) and 0-016 gram of lead nitrate (corresponding
with o-oi gram Pb) in water and making up to 500 c.c. From this a scale
of tints is prepared by diluting varying-volumcs of the liquid (e.g., i, 3, 3,
5, 10, 15, 20 c.c., etc.) to 50 c.c. and adding to each a drop of ammonium

sulphide.

Forty grams of the citric acid are dissolved in water and slight excess
of ammonia added, the cooled liquid being diluted to 500 c.c. To 50 c.c.
of this solution is added a drop of ammonium sulphide, the coloured liquid
thus obtained being compared with the above colour scale. The judging of
the colours is aided by addition of a little glyc.eri.nc>, which renders the
colours sharper and prevents the formation of precipitate. In this way
the lead content of citric acid may be- determined with great accuracy.

7.  Determination of,the Citric Acid.—In absence of other acids, it
suffices to dissolve 1-2 grams of the acid in water and to titrate the solution
with normal potassium hydroxide solution in presence of plienolphthalein ;
i c.c. of N-alkali = 0-07 grain of crystallised citric acid (C'0ljH()7 -|~ Ha(.))
and 14-5 c.c. of N-alkali — i gram of the crystallised acid.

In presence of other acids, usually oxalic, and tarlaric, these extraneous
acids must be separated and, if necessary, estimated :

(a] PRESENCE OF OXALIC ACID.—2-3 grains of the citric, acid, are dis-
solved in water and neutralised with caustic soda ; the liquid is acidified
with acetic acid and a solution of calcium sulphate, or chloride added. The
calcium oxalate formed is filtered off, washed with hot water and weighed
as carbonate or oxide. In the filtrate the citric, add is determined by pre-
cipitation as indicated under analysis of lemon juice (we. later, 4). Where
determination of the oxalic acid is unnecessary, this precipitation method
is applied directly to 2 grams of the acid.

(6) PRESENCE OF TARTARIC ACID (Allen's method), -a grams of the
citric acid are dissolved in 20 c.c. of 57% alcohol, the liquid being filtered
if necessary and made up to 45 c.c. with alcohol of the same strength ; 5
c.c. of a cold, saturated solution of potassium acetate in 57% alcohol are.
then added and the liquid shaken for 10 minutes. The presence of tartaric
acid leads to the formation of insoluble acid potassium tartnite, which may
be collected on a filter, washed first with cold saturated potassium bitar-
trate solution and then with 57% alcohol, dried at 100° and weighed, The
weight, multiplied by 0-8, or the number of c.c. of normal alkali used for
its titration, multiplied by 0-150, gives the amount of tartaric acid
contained in the 2 grams of substance taken. In the filtrate, from the
bitartrate the citric acid is determined as in lemon juice.

*
* *

Citric acid should dissolve completely in water or alcohol without leavingyeworhs should exhibit different characters according to the nature